Aluminum deposition devices and their use in spot electroplating of aluminum

ABSTRACT

A method for spot electroplating aluminum onto a metallic substrate without submersion or dipping of the metallic substrate in an electroplating bath, the method comprising: (i) spot coating said metallic substrate with an aluminum ion-containing electrolyte contained within a protective structure possessing at least one aperture, and releasing said electrolyte from said at least one aperture onto said metallic substrate to form a coating of said electrolyte thereon, wherein said electrolyte is in contact with an anode; and (ii) applying a voltage potential between the anode and metallic substrate polarized as cathode when the aluminum ion-containing electrolyte is released from said aperture and forms a coating on the metallic substrate, to produce a coating of aluminum on the substrate. Devices, such as brush and ball pen plating devices, for achieving the above-described method are also described.

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to methods andaluminum-containing electrolytes useful in the electroplating ofaluminum, and more particularly, to devices useful in spot platingaluminum onto various metallic substrates.

BACKGROUND OF THE INVENTION

Metal surface coating has played an important role in extending the lifecycle of structural materials commonly used in large rugged equipmentfor use on land, air, and sea. Aluminum and its many versatile alloysare routinely used as surface coatings for the corrosion protection ofmany metals, offering both barrier and sacrificial protection. Inaddition, aluminum and its alloys are being considered as favorablealternatives for cadmium coatings on the protective shells of electricalconnectors in military ground systems in view of the known toxic andcarcinogenic nature of cadmium and hexavalent chromium materials.

Currently, there are various methods for aluminum deposition, such ashot dipping, thermal spraying, sputter deposition, vapor deposition, andelectrodeposition. However, a particularly attractive method fordepositing aluminum and its alloys is isothermal electrodeposition,either by tank or brush plating. Electrodeposition is an attractivetechnique because it generally leads to thin, economical coatings thatare usually adherent and do not affect the structural and mechanicalproperties of the substrate. Moreover, the thickness and quality of thedeposits can be controlled by adjustment of the deposition rate bytuning such experimental parameters as overvoltage, current density,electrolyte composition, and temperature.

Unfortunately, neither aluminum nor its alloys can be electrodepositedfrom aqueous solutions because hydrogen is evolved before aluminum canbe plated. Thus, it is necessary to employ non-aqueous solvents (bothmolecular and ionic) for this purpose. On a commercial basis, aluminumis plated by using the well known SIGAL® process. Although known to bevery effective, the SIGAL® process requires a plating bath composed ofalkyl aluminum fluorides dissolved in toluene. Not surprisingly, thetechnique raises a number of environmental and safety objections becausethe alkyl aluminum compounds are pyrophoric and toxic, and the toluenesolvent is flammable and can lead to volatile organic compound (VOC)emissions. The inefficiency of aqueous electroplating also makes it amajor energy consumer. For example, in electrolytic hard chrome plating,only 10-20% of the power supplied is used for actual deposition; theremaining power is consumed through hydrogen generation and otherlosses.

More recently, aluminum-containing ionic liquids (i.e.,aluminum-containing molten salts) have gained increasing prominence assubstantially improved electrolytes for the deposition of aluminum. Theionic liquids possess an advantageous combination of physicalproperties, including non-flammability, negligible vapor pressure, highionic conductivity, and high thermal, chemical, and electrochemicalstability. Therefore, they are amenable for the electroplating ofreactive elements, which is impossible using aqueous or other organicsolvents. Thus far, the ionic liquids used for the electrodeposition ofaluminum has focused on chloroaluminate anions, which are typicallyobtained by mixing anhydrous AlCl₃ with an organic chloride salt, suchas 1-ethyl-3-methyl imidazolium chloride (EMImCl), 1-(1-butyl)pyridiniumchloride (N-BPCl), or other related salt. However, because of thehygroscopic nature of AlCl₃ and the resulting chloroaluminate, theelectroplating generally must be performed in an inert gas atmosphere,which significantly increases cost and complexity of the process.

SUMMARY OF THE INVENTION

By use of novel applicator devices and methods for their use, theinstant disclosure overcomes the persistent problem in the art of havingto implement costly precautions against moisture during aluminumelectroplating. The invention achieves this by employing applicatordevices that include a protective structure within which an aluminumion-containing electrolyte is incorporated, with the provision ofapertures in the protective structure to permit release of theelectrolyte onto a metallic substrate. The applicator device can be, forexample, a polymer membrane, a brush plating device, or a ball penplating device in which the electrolyte is (or can be) impregnated orincorporated.

In the electroplating method, the electrolyte is made to be in contactwith an anode in the device at the time the electrolyte is released fromthe protective device and applied as a coating on the substrate. Avoltage potential is then applied between the anode and the substrate(polarized as cathode) in order to produce a coating of aluminum withinan area bounded by the coating of the electrolyte. A further advantageof the methods described herein is the ability to spot electroplatemetallic substrates that are generally too large or cumbersome toelectroplate by immersing or dipping into a bath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. A general schematic depicting an exemplary electroplatingprocess using a polymer membrane, such as an ionogel or polymer gelmembrane, impregnated with an aluminum-containing electrolyte.

FIG. 2. A general schematic depicting a portable plating brush for spotelectroplating aluminum.

FIGS. 3A, 3B. Graphs showing cyclic voltammograms of a) AlCl₃-EMIC(1.5:1) and b) AlCl₃-4-propylpyridine (1.4:1) molten mixtures on a Ptelectrode (2 mm in diameter) under a scan rate of 100 mV/s at roomtemperature with Al wire used as the counter and the referenceelectrode, wherein EMIC is an acronym for ethylmethylimidazoliumchloride.

FIGS. 4A, 4B. Graphs showing cyclic voltammograms of mixtures of AlCl₃and acetamide at a) 1:1 and b) 1.2:1 ratios on a Pt working electrode (2mm in diameter) with Al wire as the counter and reference electrode. Thescan rate was 10 mV/s.

FIGS. 5A, 5B, 5C, and 5D. Graphs showing cyclic voltammograms of amixture of AlCl₃ and 4-propylpyridine (1.5:1) in a) no solvent; b)dichloromethane (DCM); c) acetonitrile (AN), and d) tetrahydrofuran(THF) on a Pt working electrode (2 mm in diameter) with Al wire ascounter and reference electrode. The scan rate was 100 mV/s.

FIG. 6. Photo showing an acrylamide polymer membrane containing 60 wt %of AlCl₃-EMIC (1.5:1).

FIG. 7. Graph showing cyclic voltammograms of the polymer membranecontaining 60 wt % of 1.5:1 AlCl₃-EMIC molten mixture at 40° C. at ascan rate of 100 mV/s. Cu and Al plates were used as working and counterelectrode, respectively. The area of the working electrode was 3.75 cm².

FIGS. 8A, 8B. Graphs showing cyclic voltammograms of a polymer membranecontaining 60 wt % of a) AlCl₃-4-propylpyridine (1.4:1) and b)AlCl₃-acetamide (1.2:1) at 40° C. at a scan rate of 100 mV/s. Cu and Alplates were used as working and counter electrode, respectively. Thearea of the working electrode was 1.68 cm².

FIG. 9. Photo showing a portable plating brush electroplating a coatingof aluminum on a copper substrate.

FIGS. 10A, 10B. Photos showing (a) macroscopic view of the aluminumcoating produced by the portable plating brush shown in FIG. 9 usingAlCl₃-EtMeImCl ionic liquid electrolyte, and (b) optical micrograph(500× ) of the same aluminum coating, wherein EtMeImCl refers to1-ethyl-3-methylimidazolium chloride.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the invention is directed to devices useful for spotelectroplating a metallic substrate (i.e., “substrate”) with an aluminumcoating. The devices include a protective structure in which thealuminum ion-containing electrolyte (i.e., “electrolyte”) is contained.The term “protective”, as used herein, indicates an ability of theprotective structure to provide substantial protection from air, andparticularly, moisture, as commonly found in air. Thus, to be optimallyprotective, the protective structure should ideally be capable ofsubstantially or completely surrounding or encasing the electrolytehoused therein, except that the protective structure includes at leastone aperture to permit release of the electrolyte onto a substrate. Foroptimal effect, the number of apertures are ideally limited to theextent possible while permitting suitable release of the electrolyte.Typically, the electrolyte will remain contained within the protectivestructure unless an action effecting release is taken. The actioneffecting release may be, for example, the application of pressure tothe device as provided by, for example, pressing or other means forapplying of pressure. The action effecting release may alternatively beprovided by including a releasing (i.e., transfer) feature in thedevice, wherein the releasing feature serves to transfer the electrolytefrom inside of the protective structure to the substrate by, forexample, capillary action or other spreading mechanism. The device, whenultimately assembled, necessarily includes electrical wiring means topermit a voltage potential and current to be transmitted between theanode and substrate polarized as cathode.

The protective structure can be made of any material non-reactive withthe electrolyte. Some materials suitable for the protective structureinclude, for example, plastic, metal, glass, or ceramic, provided thatthe material is appropriate for the intended means for release.

The device, when ultimately assembled, also includes an anode located ina position suitable for contact with the electrolyte when theelectrolyte is incorporated into the device. The anode can be any of theanodes well known in the art for electroplating aluminum. In oneembodiment, the anode is an aluminum anode. In another embodiment, theanode is an inert anode, such as a porous or non-porous graphite,titanium-containing, tantalum-containing, or platinum-containing anode.

In a first embodiment, the protective structure is a porous polymermembrane. the pores in the membrane serve as the at least one aperturedescribed above. The term “membrane”, as used herein, refers to a shapehaving two of its dimensions significantly larger (typically, at least10, 20, 50, or 100 times) than the third dimension, which can bereferred to as the thickness. Thus, the term “membrane” may adopt theshape of a film or a sheet. The membrane can have any suitablethickness. In different embodiments, the thickness is precisely, about,at least, greater than, up to, or less than, for example, 1, 2, 5, 10,20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800,900, or 1000 microns (i.e., 1000 μm, where 1000 μm is equivalent to 1mm). The thickness may also be significantly larger than 1 mm, such as2, 3, 4, 5, 6, 7, 8, 9, or 10 mm (1 cm). The thickness may also bewithin a range bounded by any two of the foregoing values. The term“about”, as used herein, generally indicates no more than ±10, ±5, ±2,or ±1% from an indicated value. In some embodiments, the polymermembrane is part of a layered or laminate structure in which the polymermembrane is in contact with or bonded to an anode layer, such as a layer(e.g., sheet or foil) of aluminum, aluminum alloy, or inert anodematerial. A portion of the polymer membrane should be left uncovered topermit the uncovered portion of the polymer membrane to make contactwith the substrate. For example, the anode layer can be in contact withor bonded with one side of the polymer membrane with the other side ofthe polymer membrane uncovered.

The pores in the polymer membrane are of suitable size to release theelectrolyte material at an acceptable rate. In a first embodiment, thepolymer membrane includes macropores, which are typically pores having asize (typically diameter, for circular pores) of above 50 nm. Indifferent embodiments, the macropores have a size of precisely, about,at least, or greater than 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90nm, 95 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170nm, 180 nm, 190 nm, 200 nm, 225 nm, 250 nm, 275 nm, 300 nm, 325 nm, 350nm, 375 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 550 nm, 600 nm, 650nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm (1 μm), 2μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 100 μm, 200 μm, 300 μm, 400μm, or 500 μm, or a particular size, or a variation of sizes, within arange bounded by any two of the foregoing values. In a secondembodiment, the polymer membrane includes mesopores, which are typicallypores having a size of at least 2 nm and up to 50 nm. In differentembodiments, the mesopores have a size of precisely or about 2 nm, 2.5nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm,6 nm, 6.5 nm, 7 nm, 7.5 nm,8 nm, 8.5 nm, 9 nm, 9.5 nm, 10 nm, 11 nm, 12 nm, 15 nm, 20 nm, 25 nm, 30nm, 35 nm, 40 nm, 45 nm, or 50 nm, or a particular size, or a variationof sizes, within a range bounded by any two of the foregoing values. Ina third embodiment, the polymer membrane includes micropores, which aretypically pores having a size of less than 2 nm. In differentembodiments, the micropores have a size of precisely, about, up to, orless than 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6,1.7, 1.8, or 1.9 nm, or a particular size, or a variation of sizes,within a range bounded by any two of the foregoing values. In someembodiments, only one type of pore is included in the polymer membrane,or one or more types of pores may be excluded. In other embodiments, twoor three of any of the types of pores are included, e.g., macropores andmesopores; or mesopores and micropores; or macropores, mesopores, andmicropores. When two or more pore types are present, the pore sizes maybe distributed within an overlapping range bounded by any two of theexemplary pore sizes provided above. Moreover, the distribution of poresizes may be within a single pore size distribution (i.e., monomodal) orbe within more than one pore size distribution (e.g., bimodal ortrimodal), as typically characterized by a pore volume vs. pore sizedistribution plot.

The polymer membrane can have any polymeric composition, provided thatit is substantially unreactive with all components of the electrolyteand can function to retain the electrolyte and release the electrolyteas intended. For most applications, the polymer is preferably flexibleto the extent that it can closely follow and make consistent contactwith the contours of the substrate surface on which it is being applied.In some embodiments, the polymer membrane may be required to bend orwrap around non-planar portions of the substrate. The polymer membranecan be constructed of, for example, a vinyl-addition polymer,polyalkylene oxide (e.g., polyethylene oxide or polypropylene oxide orco-polymer thereof), polyurethane, polyester, or polyurea. In otherembodiments, the polymer may include a substantial inorganic component,such as found in the sol gels, polysiloxanes (e.g.,polyorganosiloxanes), and hybrid organic-inorganic materials. In yetother embodiments, the polymer may be an ionogel, including those basedon ionic liquid hybrid materials, as described, for example, in Chen etal., Applied Materials & Interfaces, vol. 6, pp. 7840-7845, 2014; LeBideau, et al., Chem. Soc. Rev., 40(2):907-25, Feb. 2011; Neouze et al.,Chem. Mater., 18(17), pp. 3931-3936, 2006; and U.S. Pat. No. 8,163,834,the contents of which are herein incorporated by reference in theirentirety.

In particular embodiments, the polymer membrane is constructed, at leastpartially, of a vinyl-addition polymer. The term “vinyl-additionpolymer”, as used herein, refers to any of those polymers, known in theart, derived from the addition polymerization of unsaturated monomers.Some examples of vinyl-addition polymers include, polyacrylamide,polyethylene, polypropylene, polyvinylpyridine, polyacrylate,polymethacrylate, polybutadiene, polyacrylonitrile, polystyrene, andfluorinated versions (e.g., polyvinylidene fluoride andpolyhexafluoropropylene). In some embodiments, a homopolymer is used,while in other embodiments a copolymer is used, wherein the copolymercan include two, three, or more different monomers, and can be arranged,for example, as a block, alternating, graft, or periodic copolymer.

Numerous porous polymer membranes are known in the art, with many ofthem suited to function as materials that can be impregnated withaluminum-containing electrolytes for purposes of the instant invention.Alternatively, the porous polymer membrane can be prepared by methodswell known in the art, such as by polymerization in the presence of atemplating agent or volatile or porogenic substance. Drying and/orheating may be employed to aid in the production of pores. In someembodiments, the electrolyte is incorporated into the porous polymermembrane by impregnating the porous polymer membrane with theelectrolyte, e.g., by absorption, which typically involves capillaryaction. In other embodiments, the electrolyte is incorporated into theporous polymer membrane by forming the porous polymer membrane from areaction solution that includes the monomers and the electrolyte. In thelatter case, the electrolyte becomes entrapped within spaces of thepolymer as the polymer is being formed from the monomers.

In another embodiment, the protective structure is an applicator devicethat includes (i) a compartment in which the aluminum ion-containingelectrolyte and the anode is contained, and (ii) transferring means fortransferring the electrolyte from the compartment through the at leastone aperture onto the metallic substrate. The compartment can be of anysuitable size and shape, provided that it can hold a reservoir of theelectrolyte. The compartment can be made of any suitable material, suchas any of those described above for the protective structures. Thetransferring means is any physical feature incorporated into orintegrated with the compartment that can transfer electrolyte fromwithin the compartment to an area external from the compartment and ontoa substrate. Since a voltage potential needs to be established acrossthe substrate to the anode via the coating of electrolyte, thetransferring means should be capable of transferring the electrolyte bya process in which the transferring means makes direct contact with thesubstrate on which the electrolyte is being applied. If the transferringmeans works by indirect application (e.g., by spraying), a voltagepotential cannot be made between the cathodic substrate and anode. Forthis reason, spraying may not be considered. However, spraying may beconsidered if the coating of electrolyte made by spraying issubsequently made to be in electrical communication with the anode sothat a voltage potential between the anode and cathodic substrate can beestablished. In some embodiments, the electrolyte may be rendered highlyviscous (e.g., by inclusion of viscosity enhancing agents or hardeningagents) so as to form an adherent coating (i.e., retained film) thatmaintains its shape with minimal spreading over time, and the adherentcoating subsequently contacted with an anode in interconnection with thecathodic substrate to form a coating of aluminum. Such adherent coatingmay be applied by, for example, spraying or painting (e.g., brushing orrolling). The coating may also include a component (e.g.,photoresponsive ionogel or crosslinkable agent) that may function toharden the coating upon exposure to a stimulus, such as by irradiationor chemical treatment.

In one embodiment, the transferring means works by transferring theelectrolyte by capillary action. Such transferring means may include,for example, fibers (e.g., filaments or strands), which may or may notbe hollow. When the electrolyte is placed in the compartment, one end ofthe fibers should be in contact with the electrolyte, with the remaininglength of the fibers traversing the aperture and extending to a regionoutside of the compartment. In another embodiment, the transferringmeans may be a soft foam material capable of becoming impregnated orsaturated with the electrolyte. As the electrolyte can be transferredonto a substrate by brushing with fibers or a foam saturated withelectrolyte, the above-described applicator device can be referred to asa “brush plating device”. In some embodiments, the applicator devicerelies only on the passive transferring means (e.g., brushing) totransfer the electrolyte. In other embodiments, the applicator devicefurther includes an active transferring means to aid in transfer of theelectrolyte through the passive transferring means. The activetransferring means can be, for example, a pumping element that serves toapply pressure on the electrolyte reservoir to encourage movement of theelectrolyte into the passive transferring means.

In another embodiment, the transferring means works by transferring theelectrolyte by active spreading. The active spreading is achieved byusing mechanical action to move a transferring element in such a mannerthat movement of the transferring element transfers electrolyte from thecompartment reservoir to an area outside the compartment. The spreadingmeans can be, for example, a rotatable ball traversing an aperture inthe compartment, wherein the rotatable ball is in contact withelectrolyte in the compartment and can transfer electrolyte to a regionoutside of the compartment by being rotated, e.g., as in a ball pointpen. The ball may be non-porous, in which case only the surface of theball functions to spread the electrolyte. Alternatively, the ball may beporous, in which case the interior and surface of the ball function totransfer and spread the electrolyte. As the electrolyte can betransferred onto the substrate by contacting the ball with the substrateand employing ball rotation (e.g., by friction), the above-describedapplicator device can be referred to as a “ball pen plating device”. Inanother embodiment, the transferring means may be a roller, with amechanism simulating a paint roller, instead of a ball. The roller maybe porous or non-porous, as described above for the rotatable ball. Inthe case of a roller, as the electrolyte can be transferred onto thesubstrate by contacting the roller with the substrate and rolling theroller (e.g., by friction), the above-described applicator device can bereferred to as a “roll plating device”. As in the case of the brushplating device, the ball pen or roll plating device may or may not workin concert with an active transfer element, such as a pumping element,to improve transfer of the electrolyte to the ball or roller element.

The aluminum ion-containing electrolyte can be any of the liquidaluminum-containing electrolytes known in the art useful inelectroplating a layer of aluminum onto a metallic substrate. In orderto conduct electrical current between the anode and substrate, theelectrolyte should be suitably conductive. In a first embodiment, theelectrolyte includes aluminum ions and counterions dissolved in anon-aqueous solvent, e.g., an alkyl aluminum or aluminum halide compounddissolved in an organic (non-aqueous) solvent, such as an alkyl aluminumfluoride dissolved in toluene (as in the SIGAL® process) or use ofanother organic solvent, such as benzene, cyclohexane, tetrahydrofuran,or dimethyl sulfide. Such electrolytes are described, for example, inU.S. Pat. Nos. 4,003,804, 4,032,413, 4,071,415, 4,126,523, 4,152,220,4,379,030, 4,721,656, and U.S. Application Publication No. 2011/0253543,the contents of which are herein incorporated by reference in theirentirety. In a second embodiment, the electrolyte includes analuminum-containing ionic liquid (i.e., as an aluminum-containing moltensalt or solution of ionic liquid in a solvent). The aluminum-containingionic liquid can be those, well known in the art, which includechloroaluminate anions, such as those obtained by mixing anhydrous AlCl₃with an organic chloride salt, such as 1-ethyl-3-methyl imidazoliumchloride (EMImCl), 1-(1-butyl)pyridinium chloride (N-BPCl), or otherrelated salt. Such ionic liquid electrolytes are described, for example,in U.S. Pat. Nos. 2,446,331, 5,041,194, 5,827,602, 7,915,426, 8,778,163,U.S. Application Publication No. 2012/0189778, and Q. Liao, et al., I.Electrochem. Soc., vol. 144, no. 3, March 1997, the contents of whichare herein incorporated by reference in their entirety.

In particular embodiments, the electrolyte is exclusively or includes anionic liquid composition containing a trihalo aluminum (III) speciescomplexed with at least one organic uncharged (neutral) ligand (alsoreferred to as “ligand”). The halogen atoms in the trihalo aluminum(III) species can be selected from any of the halogens, i.e., fluorine,chlorine, bromine, and iodine, which respectively correspond to aluminumfluoride (AlF₃), aluminum chloride (AlCl₃), aluminum bromide (AlBr₃),and aluminum iodide (AlI₃), and multiples thereof, such as the dimerAl₂Cl₆. Thus, the ionic liquids described above can be convenientlydescribed according to the general stoichiometric formula AlX₃.L_(n),where X is a halogen atom, L is an organic uncharged ligand, and n is aninteger of at least 1, typically 1, 2, or 3. Molecules of solvation(i.e., adducts) may or may not also be included in the formula.Multiples of the foregoing general formula (e.g., Al₂X₆.L_(2n)) are alsoembraced by the general formula. The term “complex” or “complexed”, asused herein, indicates a bonding interaction between the neutral organicligand and the aluminum ion. The association between the aluminum ionand ligand in the above-described ionic liquid is typically a dativecovalent interaction, generally between the electron-deficient aluminumion and electron-donating heteroatom in the ligand. As the ligandconsidered in the above-described ionic liquid is uncharged, there is noionic bonding between the aluminum ion and the ligand. There is,however, an ionic association between the aluminum ion and the halideatoms, which provides the ionic character of the composition.

The organic uncharged ligand particularly considered herein is orincludes a ring structure having at least three ring carbon atoms and atleast one ring heteroatom selected from nitrogen and sulfur. Indifferent embodiments, the ring heteroatoms may be selected from onlynitrogen atoms, or only sulfur atoms, or a combination of nitrogen andsulfur atoms, or a combination of nitrogen and oxygen atoms, or acombination of sulfur and oxygen atoms. The ring structure may beunsaturated (e.g., aliphatic or aromatic) or saturated. The ringstructures generally contain a total of five, six, or seven ring atoms(i.e., five-, six-, or seven-membered rings), at least three of whichare ring carbon atoms and at least one of which is a heteroatom.Generally, the ring structure includes one, two, or three ringheteroatoms.

Some examples of five-membered unsaturated rings containing at least onering nitrogen atom include pyrrole, imidazole, pyrazole, oxazole,isoxazole, thiazole, and the triazole rings (i.e., 1,2,3-triazole and1,2,4-triazole). Some examples of six-membered unsaturated ringscontaining at least one ring nitrogen atom include pyridine, pyrazine,pyrimidine, pyridazine, 1,3,5-triazine, and oxazine rings. Some examplesof seven-membered unsaturated rings containing at least one ringnitrogen atom include azepine and the diazepine rings (e.g.,1,2-diazepine, 1,3-diazepine, and 1,4-diazepine).

Some examples of five-membered saturated rings containing at least onering nitrogen atom include pyrrolidine, imidazolidine, oxazolidine, andthiazolidine rings. Some examples of six-membered saturated ringscontaining at least one ring nitrogen atom include piperidine,piperazine, morpholine, and thiomorpholine rings. Some examples ofseven-membered saturated rings containing at least one ring nitrogenatom include azepane and diazepane rings.

Some examples of unsaturated rings containing at least one ring sulfuratom include thiophene, thiazole, isothiazole, and thiadiazole rings.Some examples of saturated rings containing at least one ring sulfuratom include tetrahydrothiophene and thiopyran rings.

The ring structure containing the at least one heteroatom may or may notalso be fused to another ring, thereby resulting in a fused ringstructure. Some examples of fused ring structures include indole,purine, quinoline (benzopyridine), isoquinoline, benzimidazole,benzoxazole, benzothiazole, benzoxazoline, benzothiophene, benzoxazine,and phenoxazine.

In some embodiments, the ring structure of the uncharged ligand includesat least one alkyl substituent (i.e., alkyl group) containing at leastone carbon atom. The alkyl substituent can improve the properties of theionic liquids, particularly by decreasing their melting points, andpreferably making them room temperature ionic liquids. In differentembodiments, the alkyl group can include precisely or at least one, two,three, four, five, six, seven, eight, nine, ten, eleven, or twelvecarbon atoms, or a number of carbon atoms within a range bounded by anytwo of the foregoing numbers. The alkyl group can be straight-chained orbranched. Some examples of straight-chained alkyl groups include methyl,ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl,n-decyl, n-undecyl, and n-dodecyl groups. Some examples of branchedalkyl groups include isopropyl, isobutyl, sec-butyl, t-butyl, isopentyl,neopentyl, 2-methylpent-1-yl, 3-methylpent-1-yl, isohexyl, isoheptyl,and isooctyl groups.

In other embodiments, the ring structure of the uncharged ligandincludes at least one alkenyl substituent (i.e., alkenyl group)containing at least two carbon atoms and the presence of at least onecarbon-carbon double bond. The alkenyl substituent can also improve theproperties of the ionic liquids, particularly by decreasing theirmelting points, and preferably making them room temperature ionicliquids. In different embodiments, the alkenyl group can includeprecisely or at least two, three, four, five, six, seven, eight, nine,ten, eleven, or twelve carbon atoms, or a number of carbon atoms withina range bounded by any two of the foregoing numbers. The alkenyl groupcan be straight-chained or branched. Some examples of straight-chainedalkenyl groups include vinyl, propen-1-yl (allyl), 3-buten-1-yl(CH₂═CH—CH₂—CH₂—), 2-buten-1-yl (CH₂—CH═CH—CH₂—), butadienyl,4-penten-1-yl, 3-penten-1-yl, 2-penten-1-yl, 5-hexen-1-yl,6-hepten-1-yl, and the like. Some examples of branched alkenyl groupsinclude propen-2-yl, 1-buten-3-yl (CH₂═CH—CH.—CH₃), 1-buten-2-yl(CH₂═C.—CH₂—CH₃), 1-penten-4-yl, 1-penten-3-yl, 2-penten-4-yl,2-penten-3-yl, and 1,4-pentadien-3-yl.

In some embodiments, the at least one alkyl or alkenyl group attached tothe ring structure is composed of only carbon and hydrogen atoms. Inother embodiments, the alkyl or alkenyl group may include one or moreheteroatoms, such as one or more selected from oxygen, nitrogen, sulfur,and halogen atoms. A particular example of an alkyl group substitutedwith at least one heteroatom is an alkyl group containing at least oneoxygen atom, e.g., a hydroxy group (OH), or ether group (—O—) as foundin the alkoxides (i.e., —OR, where R is an alkyl group with or withoutfurther heteroatom substitution) or groups of the general formula—(CH₂)_(s)—(O—CH₂CH₂)_(t)H, where s is 0 or an integer from 1 to 12 andt is 0 or an integer from 1-12. A protic group, such as OH, should notbe present in the ionic liquid if it becomes deprotonated by othergroups in the ionic liquid or by another component in contact with theionic liquid. In some embodiments, the alkyl group may be a partially orcompletely fluorinated alkyl group, such as CF₃, or CF₂CF₃, or afluorinated sulfone, such as —SO₂F or —SO₂CF₃.

The at least one alkyl or alkenyl substituent can be included on thering structure provided that it does not result in a charged ligand. Forexample, in the case of an unsaturated ring, the alkyl or alkenylsubstituent must not be located on a ring nitrogen atom if the nitrogenatom is part of an unsaturated bond, since this would result in apositively charged ring nitrogen atom (i.e., the alkyl or alkenylsubstituent can only be located on a ring carbon atom in that case). Ifthe ring nitrogen atom is not part of an unsaturated bond (either in anunsaturated or saturated ring), then the ring nitrogen atom can bear asingle alkyl or alkenyl substituent while remaining uncharged, as longas the ring nitrogen atom is not part a fused side of a fused ringsystem. The alkyl or alkenyl substituent must not be located on a ringsulfur atom since this would result in a positively charged ring sulfuratom.

In particular embodiments, the ionic liquid is or includes analkyl-substituted or alkenyl-substituted pyridine or imidazole ring,wherein the alkyl or alkenyl substituent is on a ring carbon atom of thepyridine or imidazole ring. The alkyl-substituted pyridine ligand canbe, for example, a 2-alkyl-pyridine, 3-alkyl-pyridine, 4-alkyl-pyridine,2,3-dialkyl-pyridine, 2,4-dialkyl-pyridine, 3,4-dialkyl-pyridine,2,3,4-trialkyl-pyridine, 3,4,5-trialkyl-pyridine, or2,3,5-trialkyl-pyridine, wherein it is understood that the numberdesignating the alkyl group is relative to the location of the ringnitrogen atom, where the ring nitrogen atom is designated as position 1(thus, a 4-alkyl-pyridine contains the alkyl group in a positiondirectly opposite from the ring nitrogen atom in the pyridine ring). Insimilar fashion, the alkyl-substituted imidazole ligand can be, forexample, a 2-alkylimidazole, 4-alkylimidazole, 2,4-dialkylimidazole,4,5-dialkylimidazole, or, 2,4,5-trialkylimidazole, wherein it isunderstood that the number designating the alkyl group is relative tothe location of the ring nitrogen atoms, which occupy positions 1 and 3on the imidazole ring. The alkyl group in any of the above exemplaryalkyl-substituted pyridine or imidazole ligands can be replaced with analkenyl group to provide an equal number of exemplaryalkenyl-substituted pyridine and imidazole ligands. The ring may alsoinclude a combination of alkyl and alkenyl groups. In some embodiments,the alkyl-substituted ring contains no substituent other than one ormore alkyl and/or alkenyl substituents, i.e., remaining positions on thering are occupied by hydrogen atoms.

The ionic liquid described herein is typically a liquid at roomtemperature (e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. However, insome embodiments, the ionic liquid may not be a liquid at roomtemperature, but becomes a liquid at a higher temperature than 30° C. ifit is used at an elevated temperature that melts the compound to be anionic liquid. Thus, in some embodiments, the ionic liquid may have amelting point of up to or less than 100, 90, 80, 70, 60, 50, 40, or 35°C. In other embodiments, the ionic liquid may be a liquid at atemperature of or less than 100, 90, 80, 70, 60, 50, 40, or 35° C. Inother embodiments, the ionic liquid is a liquid at or below 10, 5, 0,−10, −20, −30, or −40° C. The term “liquid”, as used herein, indicatesan ability of the substance to readily flow, typically no more thanabout 1,000 centipoise (1,000 cP). In different embodiments, theviscosity of the ionic liquid is up to or less than, for example, 1,000,800, 700, 600, 500, 400, 300, 200, 100, 50, 25, 10, 5, or 1 cP, or aviscosity within a range bounded by any two of these values.

The ionic liquids described above are generally prepared by combiningand mixing an aluminum trihalide (e.g., AlCl₃) and the organic neutralligand in the liquid state in a molar ratio that produces a compositionthat behaves as an ionic liquid at a desired temperature, such as roomtemperature. In some embodiments, the mixture is heated to ensuredissolution of the aluminum trihalide in the organic neutral ligand. Inparticular embodiments, the ratio of aluminum trihalide to organicneutral ligand is precisely or about, for example, 0.5:1, 0.6:1, 0.7:1,0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1,1.8:1, 1.9:1, or 2:1, or a ratio within a range bounded by any two ofthe foregoing values.

In one embodiment, the electrolyte (electroplating solution) containsthe ionic liquid in the substantial or complete absence of a solvent,i.e., typically as a molten form of the ionic liquid. In anotherembodiment, the electrolyte contains the ionic liquid in admixture withone or more solvents. The solvent may function, for example, to helpsolubilize other components in the electrolyte (e.g., an electrolytesalt), improve wettability, or improve qualities of the aluminumdeposit. The one or more solvents can be selected from any of theorganic and inorganic solvents known in the art, provided that thesolvent or solvent mixture does not adversely react or interact with theionic liquid or the plating process. The solvent or solvent mixtureshould be completely miscible with the ionic liquid and any othercomponents that may be included in the electrolyte.

The organic solvent can be ionic or non-ionic. In the case of an ionicsolvent, the ionic solvent can be any of the ionic liquids of the art oras described herein. In the case of a non-ionic solvent, the non-ionicsolvent can be, for example, a hydrocarbon, alcohol, ketone, carbonate,sulfone, siloxane, ether, nitrile, sulfoxide, or amide solvent, or amixture thereof. Some examples of hydrocarbon solvents include hexanes,cyclohexane, benzene, toluene, decalin, and xylenes, or halogenatedversions of hydrocarbons, e.g., methylene chloride, trichloroethylene,or perchlorethylene. Some examples of alcohol solvents include methanol,ethanol, n-propanol, isopropanol, n-butanol, and isobutanol, and thediols, such as ethylene glycol, diethylene glycol, and triethyleneglycol. Some examples of ketone solvents include acetone and 2-butanone.Some examples of carbonate solvents include propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate(DMC), and fluorocarbonate solvents (e.g., fluoroethylene carbonate andtrifluoromethyl propylene carbonate). Some examples of sulfone solventsinclude methyl sulfone, ethyl methyl sulfone, methyl phenyl sulfone,methyl isopropyl sulfone (MiPS), propyl sulfone, butyl sulfone,tetramethylene sulfone (sulfolane), and phenyl vinyl sulfone. Someexamples of siloxane solvents include hexamethyldisiloxane (HMDS),1,3-divinyltetramethyldisiloxane, the polysiloxanes, andpolysiloxane-polyoxyalkylene derivatives. Some examples of ethersolvents include 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,3-dioxolane,tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, diglyme,triglyme, 1,3-dioxolane, and the fluorinated ethers (e.g., mono-, di-,tri-, tetra-, penta-, hexa- and per-fluoro derivatives of any of theforegoing ethers). Some examples of nitrile solvents includeacetonitrile, propionitrile, and butyronitrile. Some examples ofsulfoxide solvents include dimethyl sulfoxide, ethyl methyl sulfoxide,diethyl sulfoxide, methyl propyl sulfoxide, and ethyl propyl sulfoxide.Some examples of amide solvents include formamide,N,N-dimethylformamide, N,N-diethylformamide, acetamide,N,N-dimethylacetamide, N,N-diethylacetamide, gamma-butyrolactam, andN-methylpyrrolidone. Other organic solvents includehexamethylphosphoramide (HMPA) and1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU). In the caseof an inorganic solvent, the inorganic solvent is other than water, suchas carbon disulfide or supercritical carbon dioxide. In someembodiments, any one or more of the above classes or specific types ofsolvents are excluded from the electroplating solution.

If a solvent is included, the one or more ionic liquids can be includedin any suitable amount, typically at least 10 wt % by weight of solventand ionic liquid. In different embodiments, the ionic liquid is includedin an amount of precisely, about, at least, or above, for example, 10,15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 95, 98, or 100 wt % byweight of the ionic liquid plus solvent, or an amount within a rangebounded by any two of the foregoing exemplary values.

In some embodiments, one or more salts of an alkali or alkaline earthmetal is included in the electrolyte to increase the conductivity of theelectrolyte or to improve aspects of the aluminum deposit. The saltshould be completely dissolved in the electrolyte at the temperatureemployed for electroplating. The salt can be, for example, a halide ofan alkali or alkaline earth metal. Some examples of alkali halidesinclude lithium chloride, lithium bromide, sodium fluoride, sodiumchloride, sodium bromide, potassium chloride, and potassium bromide.Some examples of alkaline earth halides include magnesium chloride,magnesium bromide, and calcium chloride. The salt can be included in anydesired amount in the electrolyte to suitably adjust the conductivity ofthe electrolyte or other aspects of the process. In some embodiments,the salt is included in an amount of 0.1, 0.5, 1, 2, 5, 10, 15, or 20 wt% by weight of the electrolyte, or in an amount within a range boundedby any two of the foregoing values.

In another aspect, the instant disclosure is directed to methods forelectroplating aluminum onto a metallic substrate by use of any of thespot plating applicator devices described above. The term “spotelectroplating” (or equivalently, “spot plating”), as used herein, ismeant to indicate a process of electroplating in which the substrate isnot submerged or dipped into an electroplating bath, and instead,electroplated in one or more regions of the substrate outside of anelectroplating bath. The one or more regions of the substrate beingplated each typically define a surface area less than the total surfacearea of the substrate. However, the instant invention contemplates thepossibility where a substantial or total platable surface area of asubstrate is aluminum plated by the spot plating process disclosedherein, with a result similar or commensurate to what would be providedby the substrate being submerged in an aluminum electroplating bath.

In the method, any of the applicator devices described above, chargedwith aluminum-containing electrolyte, is manipulated to release theelectrolyte to form a coating of the electrolyte on the metal substrate.Before, during, or after a coating of the electrolyte is formed on thesubstrate, a suitable voltage potential is maintained across the anodeand the substrate polarized as cathode in order to convert theelectrolyte coating into a coating of aluminum. To achieve this, theanode should be in contact with the electrolyte (typically, in contactwith the electrolyte reservoir, but possibly in direct contact with thecoating). In some embodiments, as applied to those devices containingcompartments, the anode is at least partially submerged within theelectrolyte contained in the compartment.

In the case of an electrolyte-impregnated polymer membrane, the polymermembrane may be contacted with the substrate and suitably pressed torelease the electrolyte from pores therein to form a coating on thesubstrate. In some embodiments, the polymer membrane includes fasteningmeans (e.g., tape, or hook and loop fastener, such as Velcro®) to keepthe polymer membrane firmly applied onto the substrate (e.g., bywrapping onto itself) and to compress the polymer membrane to encourageegress of the electrolyte. In some embodiments, the polymer membrane hasa sticky or tacky quality that keeps it firmly affixed to the substrate.

Before or after a coating has been formed, and with the polymer membranestill in contact with the electrolyte coating, a voltage potential isapplied between an aluminum anode (e.g., aluminum foil), which is incontact with the polymer membrane (typically, the side opposite to theside in contact with the substrate), and the metallic substratepolarized as cathode to form a coating of aluminum on the substrate. Thealuminum coating is necessarily within or defined by the area bounded bythe coating of electrolyte. The polymer membrane can then be removed toreveal the freshly coated layer of aluminum. If necessary, the polymermembrane can be recharged with electrolyte before being used to spotplate a different, overlapping, or same section of the substrate, orbefore spot plating a different substrate.

In the case of a brush plating device, the fibers of the brush, oncesaturated with electrolyte by capillary action or by application ofpressure (e.g., by a pump on the electrolyte reservoir), are contactedwith the substrate to deposit a coating of electrolyte on the substrate.With the fibers in contact with the electrolyte coating, a voltagepotential is applied between the anode (in contact with the electrolytereservoir) and the metallic substrate polarized as cathode to form acoating of aluminum on the substrate.

Similarly, by use of a ball pen or roller plating device, electrolyte ismade to coat or saturate the ball or roller along with suitablemechanical action to transfer electrolyte from the ball or roller ontothe substrate to deposit a coating of electrolyte on the substrate. Withthe ball or roller in contact with the electrolyte coating, a voltagepotential is applied between the anode (in contact with the electrolytereservoir) and the metallic substrate polarized as cathode to form acoating of aluminum on the substrate.

The metallic (conductive) substrate can have any composition for whichdeposition of aluminum may be desired. The metallic substrate mayinclude, for example, one or more metals selected from titanium,tantalum, iron, cobalt, nickel, copper, and zinc, and thus, may be asubstantially pure metal or a binary, ternary, or higher alloy. Inparticular embodiments, the metallic substrate is iron, or aniron-containing alloy, such as a steel.

The electroplating process can employ any of the conditions (e.g.,temperature, concentration, voltage, current density, etc.) commonlyused in the art of aluminum electroplating, provided that the conditionsare suitably adjusted and modified, if necessary, to accommodate thenovel electroplating processes and/or aluminum-containing electrolytesdescribed herein. The conditions can be as disclosed, for example, inU.S. Pat. Nos. 4,003,804, 4,071,415, 4,126,523, 4,152,220, 4,379,030,and 5,041,194, the contents of which are herein incorporated byreference in their entirety.

In some embodiments, the spot electroplating process is conducted in airwithout alteration of the atmosphere. In other embodiments, theelectroplating process is conducted under a modified atmosphere, whichcan be partially or completely composed of an inert gas. The inert gasmay be, for example, nitrogen or argon. The use of an inert gas may behelpful in preventing or lessening exposure of the electrolyte tomoisture and oxygen. In the case of a brush, ball pen, or roller platingdevice, in some embodiments the compartment housing the electrolyte isinitially or repetitively flushed with an inert gas to further ensureprotection of the electrolyte from air.

In some embodiments, the electroplating process is conducted with theelectrolyte being at or below room temperature, e.g., a temperature ofabout, up to, or less than 15, 20, 25, or 30° C. In other embodiments,the electroplating process is conducted with the electrolyte being at anelevated temperature, such as a temperature of about, at least, or above40, 50, 60, 70, 80, 90, 100, 110, or 120° C. In other embodiments, theelectroplating process is conducted with the electrolyte being attemperature within a range bounded by any two of the foregoing exemplarytemperatures. Any of the applicator devices described above can beconfigured to include a heating or cooling element either inside oroutside of the compartment (and optionally, a temperature measuringdevice) to achieve an electrolyte temperature lower or higher thanambient temperature. Alternatively, or in addition, the substrate may besuitably heated or cooled to a desired temperature before, during, orafter coating the substrate with the electrolyte.

The electroplating process may use direct or pulse current. Any suitablecurrent density may also be used, such as a current density of at least0.01, 0.05, 0.1, 0.5, or 1 A/dm² and up to 2, 5, 10, 15, 20, 25, 30, 40,or 50 A/dm². The electroplating time may be suitably varied and used inconjunction with a particular current density and temperature to achievea desired thickness of the aluminum coating. The electroplating time maybe, for example, 1, 5, 10, 20, 30, 40, 50, 60, 90, or 120 minutesdepending on the current density and temperature to achieve a desiredthickness. The thickness of the aluminum coating for the initial (firstplate) or final (i.e., total of one or successively layered plates) maybe precisely, about, at least, above, up to, or less than, for example,1, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, or 300microns, or a thickness within a range bounded by any two of theforegoing values.

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

EXAMPLES Introduction

By one methodology, the moisture sensitivity of chloroaluminate-basedionic liquids was significantly reduced by forming polymer gelelectrolytes (polymer membranes), either by impregnating liquidelectrolytes into preformed membranes, or by co-casting polymer andliquid electrolytes, or by copolymerization of monomers in the presenceof plasticizers. For more practical applications, the polymer gelmembranes can be cast directly onto aluminum foil, and the resultinglayered composite wrapped around a substrate to perform the plating. Inthis configuration, the aluminum foil serves as the anode and thesubstrate as the cathode during the electroplating process. A generaldepiction of the process using a polymer gel electrolyte is provided inFIG. 1.

By another methodology, the moisture sensitivity of thechloroaluminate-based ionic liquids was significantly reduced by sealingthe ionic liquid electrolyte inside the reservoir of a portable platingbrush. The device can be conveniently used when plating is desired at aparticular location of a large substrate. A depiction of the processusing a portable plating brush is provided in FIG. 2.

The above two methodologies, and variants thereof, can facilitate theelectroplating process particularly in situations where the substrate iseither too large to fit into a conventional plating bath or theconfiguration of the substrate prohibits such a plating process. Theseportable plating devices for electrodeposition of Al will benefitcoating manufacturers and electronics assembly by reducing oreliminating the use of pyrophoric and/or toxic materials while beingconvenient and integratable into conventional manufacturing processes.

Experimental Section

Preparation of a room temperature molten mixture: Ethylmethylimidazoliumchloride (EMIC) was purified by recrystallization and vacuum dryingbefore use. 4-propylpyridine was purified by distillation. Acetamide andAlCl₃ were purified by sublimation. The AlCl₃-containing mixtures wereprepared by slowly adding AlCl₃ to the imidazolium chloride (or theorganic solvent) inside an argon-filled glove box. The molar mixingratio for EMIC was fixed at an AlCl₃: EMIC ratio of 1.5:1, while thatfor 4-propylpyridine was fixed at an AlCl₃:4-propylpyridine ratio of1.4:1. For acetamide, the mixing ratio was fixed atAlCl₃:Acetamide=1.2:1.

Preparation of a self-standing membrane using AlCl₃-based moltenmixture: To a cooled solution of AlCl₃ in dichloromethane was slowlyadded an equal molar amount of acrylamide with stirring. After addition,the mixture changed to a clear yellow solution. Under the protection ofnitrogen, a calculated amount of AlCl₃-based molten mixture was added tothis solution, followed by addition of the initiator AIBN (1 wt % ofmonomer). Finally, the solution was cast into a Teflon®-coated aluminumPetri dish and evaporated at room temperature to result in aself-standing membrane after 24 hrs.

Preparation of portable plating brushes for the electrodeposition of Al:The portable plating brush was made by inserting a short length offiberglass rope into the end of plastic or glass tubing so as to providea brush-type tip. Lodging a ceramic ball into the end of the tubeprovided a ball pen. A suitable amount of the AlCl₃-EtMeImCl ionicliquid was added into the barrel/reservoir of the brushes, which soakedinto the rope by capillary action or was retained as a liquid in theballpoint pen. Leaking of ionic liquid from the brush was avoided byusing a high-density rope, with care taken to not add an excessiveamount of electrolyte. For the ball pen, the ceramic ball was retainedloosely to permit it to rotate and provide fresh electrolyte. However, aplug of potting epoxy was cast into the tube barrel and drilled with asmall orifice to allow the ionic liquid to flow slowly around the balltip without draining quickly from the reservoir.

The anode used was a large spiral of high purity 1-mm diameter aluminumwire or other source of pure Al. To incorporate the anode, the anode wasforced into the end of the rope inside the barrel of the brush orimmersed in the liquid contents of the ball pen. Because the deviceswere being tested under ambient conditions in the open atmosphere, theywere flooded with an inert shielding gas (dry nitrogen or even very dryair is sufficient) to exclude moisture.

Electrochemical measurement: Cyclic voltammetry (CV) was performedinside an argon-filled glove box under different scan rates. For thethree-electrode system, Pt was used for the working electrode. Al wasused as counter and reference electrode. For the polymer gel membranes,a two-electrode system was used system in which copper (Cu) plate wasused as the working electrode and Al plate as the anode. Platinum wastreated by polishing with Al₂O₃, followed by washing with deionizedwater and drying. The copper electrode was treated by polishing withsand paper, followed by degreasing in acetone under ultrasonic exposurefor 15 minutes, and then activated in 5 wt % HCl aqueous solution fortwo minutes to remove any oxide layer that may have formed. Finally, thecopper electrode was rinsed thoroughly with deionized water anddegreased in dichloromethane for 10 minutes to remove organic impuritiesand form a chloride layer resistant to oxide formation. The Al electrodewas treated by polishing with sand paper, followed by activation in anacidic solution composed of 1% HNO₃, 65% H₃PO₄, 5% acetic acid, andwater for 5 minutes. The Al electrode was then rinsed thoroughly withdeionized water and degreased in acetone for 5 minutes.Controlled-current electrolysis experiments were performed with a simpleadjustable DC power supply.

Results and Discussion

Cyclic voltammetry of molten mixtures: FIGS. 3A and 3B show the cyclicvoltammograms of a) AlCl₃-EMIC (1.5:1) and b) AlCl₃-4-Propylpyridine(1.4:1) molten mixtures on a Pt electrode (2 mm in diameter) under ascan rate of 100 mV/s at room temperature. In both mixtures, thereduction peaks attributed to Al deposition and the oxidation peaksattributed to the stripping of Al were observed. The overpotential fordeposition of Al in EMIC mixture was observed to be −100 mV while thatin 4-propylpyridine was observed to be −125 mV, which indicates that theformer (EMIC mixture) is more kinetically favorable for aluminumdeposition. In addition, by comparing these two CVs, it is found thatthe current densities of the EMIC-based mixture are significantly higherthan the current densities of the 4-propylpyridine-based mixture, whichindicates a much higher ionic conductivity in the former solution.

FIGS. 4A and 4B show the CVs of the mixtures of AlCl₃ and acetamide atdifferent ratios (1:1 and 1.2:1 AlCl₃ to acetamide) on a Pt workingelectrode (2 mm in diameter) under a scan rate of 10 mV/s. Al wire wasused as the counter and reference electrode. Under similar conditions asused for the EMIC- and 4-propylpyridine-based mixtures, the depositionand stripping of Al were observed for these two mixtures. However, thecurrent densities were found to be much higher for the 1.2:1 mixturethan for the 1:1 mixture, which indicates more cation complex[AlCl₂(Acetamide)₂]⁺ in the former solution. In addition, theoverpotential for Al deposition was found to be only −80 mV for the1.2:1 mixture while it was found to be −180 mV for the 1:1 mixture,suggesting that a higher amount of AlCl₃ is more favorable for Aldeposition.

In order to cast the polymer gel electrolyte membrane, a suitable oroptimal solvent was first sought. To do this, different solvents wereadded to the mixture of AlCl₃-4-propylpyridine (1.5:1), and the CV wasscanned at 100 mV/s at room temperature. FIGS. 5A-5D show the comparisonof the CVs. The results obtained without solvent (FIG. 5A) indicate awell defined Al deposition and stripping peak. With addition ofdichloromethane (DCM), as shown in FIG. 5B, the current densities becamesignificantly increased, which indicates a much improved ionicconductivity due to reduced viscosity. However, when acetonitrile (AN)was added to the mixture, as shown in FIG. 5C, it became separated intotwo layers, which indicates that the coordination between AN and AlCl₃is much stronger than between AN and propylpyridine, and as a result noreversible Al deposition/stripping was observed. The same result wasobserved when tetrahydrofuran (THF) was used as solvent, as shown inFIG. 5D. Thus, when preparing polymer gel membranes, DCM was used as asolvent.

Self-standing polymer gel membranes containing AlCl₃-based moltenmixture: When AlCl₃ was mixed with acrylamide directly, a solid wasformed, indicating that a polymerization reaction occurred due to theexothermal reaction. To avoid such side reaction, AlCl₃ was added toacrylamide/DCM solution at 0° C. using an ice bath. After mixing them at0° C., AICl₃-containing mixtures were added, followed by addition ofAIBN as initiator and polymerization at room temperature for 24 hours.FIG. 6 shows a typical picture of the polymer gel membrane containing 60wt % of AlCl₃-EMIC (1.5:1) mixture.

FIG. 7 shows the CVs of the membrane containing 60 wt % of AlCl₃-EMIC(1.5:1) mixture with Cu plate as the working electrode and Al plate asthe counter electrode. Generally, the membrane exhibited goodelectrochemical behavior for the deposition and stripping of Al. It wasnoticed that the current densities increased with increasing scancycles, which indicates an activation process, probably due to theresidual surface oxide on the Al plate. Nevertheless, this was the firstexample of the membrane showing the deposition and stripping of Al. Anadditional reduction peak was observed at 0.1 V, which may be due to thereduction of residual double bond (acryl group) within the membrane.

Polymer gel membranes containing 60 wt % of AlCl₃-4-propylpyridine(1.4:1) and AlCl₃-acetamide (1.2:1) were also prepared. Thecorresponding CVs are provided in FIGS. 8A and 8B, respectively. Thedeposition and stripping peaks of Al were observed in both membranes.However, the current densities were much smaller than that based on theAlCl₃-EMIC mixture, mainly due to the intrinsic lower ionicconductivities of the latter two membranes.

Aluminum deposition using portable plating brush: FIG. 9 shows a photoof the portable brush-type pen during plating of aluminum on a coppersubstrate. When the brush was pressed against the substrate, a smallamount of ionic liquid was deposited onto the substrate. When the powersupply was active, the electrodeposition of Al began. The Al film wasproduced in a very short amount of time (about 5 seconds), and the totalplating process lasted about 10 minutes. FIG. 10A provides a photo of Aldeposited on a Cu coupon with the portable plating brush. An opticalmicroscope image of the film is shown in FIG. 10B. Visually, the Alfilms deposited on Cu have a specular appearance.

SUMMARY

In summary, polymer gel membranes containing AlCl₃-EMIC (1.5:1),AlCl₃-acetamide (1.2:1), and AlCl₃-4-propylpyridine (1.4:1) weresuccessfully prepared for the first time. These polymer gel membranesexhibited good electrochemical behavior for the deposition and strippingof Al. It has been shown that the selection of solvent in the process ofpreparation of the polymer gel membrane affects the electrochemicalproperties of the membrane, and that the intrinsic ionic conductivity ofthe ionic liquid plays a key role in the performance of the finalpolymer gel membrane. Another way to increase the electrochemicalperformance of the polymer gel membrane is to use a higher temperature.For practical applications, other polymer matrixes such aspoly(vinylidene fluoride-hexafluoropropylene) (PVdF(HFP)),polyvinylpyridine, polyacrylate, polymethacrylate, polyacrylonitrile(PAN), polyethylene oxide (PEO), polyethylene, polypropylene membranes,and the like, can also be used.

In addition, a portable plating brush using an Al-based ionic liquid(AlCl₃-EtMeImCl) electrolyte was also successfully prepared. The processwas used to successfully plate Al on Cu or steel substrates. Al filmscan be produced in a very short time (˜5 seconds), and after 5 minutesof plating time, a dense specular film of Al was obtained on Cu orsteel. The portable plating brush can find a wide variety ofapplications in various industries, including the defense industry.Moreover, the plating process can be extended to aluminum alloy plating,such as Al—Mn, Al—Nb, Al—W plating, by controlling the composition ofthe ionic liquid and with addition of the appropriate alloying metalsand other components.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A method for spot electroplating aluminum onto ametallic substrate without submersion or dipping of the metallicsubstrate in an electroplating bath, the method comprising: (i) spotcoating said metallic substrate with an aluminum ion-containingelectrolyte contained within a protective structure possessing at leastone aperture, and releasing said aluminum ion-containing electrolytefrom said at least one aperture onto said metallic substrate to form acoating of said aluminum ion-containing electrolyte onto said metallicsubstrate, wherein said aluminum ion-containing electrolyte is incontact with an anode; and (ii) applying a voltage potential between theanode and said metallic substrate as cathode when said aluminumion-containing electrolyte is released from said aperture and forms acoating on said metallic substrate, wherein said coating of electrolyteconducts electrical current between said anode and metallic substrate,to produce a coating of aluminum on said metallic substrate within anarea bounded by the coating of said aluminum ion-containing electrolyte.2. The method of claim 1, wherein said protective structure is a polymermembrane in which said aluminum ion-containing electrolyte isimpregnated, wherein said polymer membrane contains pores for releasingsaid aluminum ion-containing electrolyte onto the substrate.
 3. Themethod of claim 2, wherein said polymer membrane is laminated withaluminum foil, serving as anode, on a side of the polymer membrane notin contact with the metallic substrate.
 4. The method of claim 2,wherein said polymer membrane is constructed of a vinyl-additionpolymer.
 5. The method of claim 2, wherein said polymer membrane isremoved to reveal a freshly coated layer of aluminum within its originalbounds.
 6. The method of claim 1, wherein said aluminum ion-containingelectrolyte comprises a non-aqueous solvent in which aluminum ions andcounterions are dissolved.
 7. The method of claim 1, wherein saidaluminum ion-containing electrolyte comprises an aluminum-containingionic liquid.
 8. The method of claim 1, wherein said protectivestructure is an applicator device comprising (i) a compartment in whichsaid aluminum ion-containing electrolyte and said anode is contained,and (ii) transferring means for transferring said aluminumion-containing electrolyte from the compartment through said at leastone aperture onto the metallic substrate, wherein said transferringmeans is in contact with the metallic substrate during electrolyteapplication and voltage application.
 9. The method of claim 8, whereinsaid applicator device is a brush plating device in which saidtransferring means comprises fibers.
 10. The method of claim 8, whereinsaid applicator device is a ball pen plating device in which saidtransferring means is a rotatable ball.
 11. The method of claim 8,wherein said anode is at least partially submerged within the aluminumion-containing electrolyte.
 12. The method of claim 8, wherein saidcompartment is flushed with an inert gas.
 13. The method of claim 8,wherein said aluminum ion-containing electrolyte comprises a non-aqueoussolvent in which aluminum ions and counterions are dissolved.
 14. Themethod of claim 8, wherein said aluminum ion-containing electrolytecomprises an aluminum-containing ionic liquid.
 15. The method of claim1, wherein said metallic substrate is comprised of at least one metalselected from titanium, iron, cobalt, nickel, copper, and zinc.
 16. Adevice useful for spot electroplating a metallic substrate withaluminum, the device comprising a protective structure suitable forhousing an aluminum ion-containing electrolyte, wherein said protectivestructure possesses at least one aperture suitable for release of thealuminum ion-containing electrolyte, and wherein said protectivestructure includes an anode in a location suitable for contacting saidaluminum ion-containing electrolyte.
 17. The device of claim 16, whereinsaid protective structure houses said aluminum ion-containingelectrolyte.
 18. The device of claim 16, wherein said protectivestructure is a polymer membrane in which said aluminum ion-containingelectrolyte is impregnated, wherein said polymer membrane contains poresfor releasing said aluminum ion-containing electrolyte onto thesubstrate.
 19. The device of claim 18, wherein said polymer membrane islaminated with aluminum foil, serving as anode, on a side of the polymermembrane, with a portion of the polymer membrane not laminated withaluminum foil, said portion of polymer membrane suitable for contactwith the metallic substrate to deposit a coating of aluminum thereon.20. The device of claim 18, wherein said polymer membrane is constructedof a vinyl-addition polymer.
 21. The device of claim 16, wherein saidprotective structure is an applicator device comprising (i) acompartment suitable for housing said aluminum ion-containingelectrolyte and containing said anode, and (ii) transferring means fortransferring said aluminum ion-containing electrolyte from thecompartment through said at least one aperture onto the metallicsubstrate by direct contact of the transferring means with the metallicsubstrate.
 22. The device of claim 21, wherein said applicator device isa brush plating device in which said transferring means comprisesfibers.
 23. The device of claim 21, wherein said applicator device is aball pen plating device in which said transferring means is a rotatableball.